Aerodynamics Considerations on Greenhouses

Aerodynamics is a very complex concept in how it applies to greenhouses and similarly constructed buildings. The better that you understand these dynamics and how to deal with them, the better your chances of having long-term success with the structural integrity of your building.

In many of Norm’s presentations, he has compared the similar shape of a greenhouse to that of an airplane wing. The basic challenge is that you want the opposite result. An airplane needs to efficiently get off the ground while a greenhouse needs to stay where it is.

The greater the distance over a surface is (wing or building), the greater that the lift is. When height is combined with distance, there is a multiplier effect. That is why a pilot will extend the flaps on a wing when taking off and landing.

It is the vacuum on the backside of a structure or a wing that causes the lift. The upward force of the vacuum on the backside is actually double what the leading edge force is.

This effect happens exponentially even when a small change is made. Many people have suggested, “I am only lifting the building two feet (or some other relatively small amount)”. They do not realize that doing that small lift is sometimes adding 50% or even doubling the wind load on the building.

These forces must especially be considered when a structure is oriented so that the prevailing winds hit the sides of the building. Structurally, it is preferable to have the structure facing into the prevailing winds. If the lay of the land or the logistics of how you work in the structure do not allow this, there are ways that the stress on the building can be minimized.

The least expensive way to stiffen a building is to add crossties (also known as collar ties). Crossties tie the left and right sides of the structure together so that the load is more balanced in how it affects the structure. Reducing the hoop spacing will automatically make the structure stronger with a greater ability to resist the effects of external forces.

Ultimately, however, a structure’s ability to deal with the aerodynamics of a situation is only as good as the anchoring provided. Anchors provide stability for resisting up, down and lateral forces. There is no such thing as too many anchors.

These things add to the structural ability to handle things but it does not help the cover. Extra steps must be implemented to secure the sides of the cover. This includes but is not limited to adding extra and longer screws for the cover fastening system.

A discussion on aerodynamics is not complete without discussing windbreaks. The only effective windbreak is trees. Trees restrict the speed of the wind coming through and therefore minimize aerodynamic stresses on a building. There is an optimal distance from a windbreak. This is determined by the size, density and texture of the trees involved. Since this is something that changes with age and season, there is not a perfect formula.

One last note on windbreaks. Another building, especially a bigger building is not an effective barrier. If anything, this building will significantly increase the aerodynamic stresses on your building. Wind will be swirling in one direction and then a slight shift in wind direction will reverse the direction of the swirling.

Please note: We are not engineers and as such can not give structural opinions. The above points are simply items that should be considered and come from 40+ years of experience of watching what wind does to buildings.

Adding a Softcover Structure to the Side or End of Another Building

There are two ways of adding a structure to the side of a building. It can be done as a lean-to (half structure) that goes parallel to the building and up to the eave, or it can be a complete building at 90 degrees to the existing building. This article applies to the latter option.

When considering attaching an MSS structure at 90 degrees to another building, there are some important considerations to be mindful of before the purchase.

The first is that these buildings are almost always considered high humidity (especially when it is a greenhouse). This means that you will be subjecting that wall to a higher level of moisture. Extra waterproofing should be considered. This high level of humidity should be an extra concern if the intent is to use the warm air as a source of heat for the solid building.

The other thing to bear in mind is the potential snow shedding patterns from the bigger building roof. If the height difference is more than 2’, measures should be implemented to slow the process of shedding snow. Without slowing the rate that the snow comes off the taller building, the force of the impact could be triple or quadruple the weight of the actual snow.

If there is a likelihood of significant snow levels being shed, we recommend reducing the rib spacing of the first 12’ of the building. Going from 4’ to 3’ spacing will increase the strength by 1/3. Going from 3’ to 2’ is a 50% increase in strength. This will give your building the added strength for the impact of shedding snow and the volume that would potentially be on the roof.

The third thing that needs to be considered is how the cover will be fastened to your shelter at the wall. For a stand-alone building, you would be on a ladder or platform off the end but this is not possible if the end hoop is right against the wall.

One option is to have the first hoop about 2’ from the wall and then cover that section with something solid (plywood, sheet metal, Lexan, etc.). This will give you a place to crawl up and secure the cover into the wirelock.

Another option would have you put the first hoop about ½” to 1” from the wall. The wirelock channel would be installed on the underside of that hoop. During the cover installation, you would slide the cover through the gap and then wrap the cover around to the bottom. The wire inserts would be installed from the underside. This option is a little more tricky when doing the double plastic cover. After the cover is installed, the gap can be filled with square foam strips which are available at the building centers. Extra care must be exercised to protect the cover from bolt heads and nuts.

The third option would be to install the structure as per normal but about 1’ from the wall. The covering would be done as usual and once this is complete, the building would be slid up against the wall. This process is a bit risky since the building is not secured to the anchors for a short period. The longer the building is, the more challenging this option is.

The last challenge which needs some attention has to do with the method of ventilation which will be used. Typical ventilation flows through the building. In this scenario, ventilating though the building would also mean that you have to go through the attached building. It can be done, but you would be best to get some additional advice on the process.

If roll-up sides are going to be used, it must be noted that the attached building will interfere with proper airflow.

The challenge with using forced ventilation is “where does the air get into the building. It would be best to create a sketch of the building with thoughts as to what you intend to do. We will use our experience to advise you.

It is important to understand and work through these challenges before you purchase. We are here to advise.

Aerodynamics Considerations on Greenhouses

Aerodynamics is a very complex concept in how it applies to greenhouses and similarly constructed buildings. The better that you understand these dynamics and how to deal with them, the better your chances of having long-term success with the structural integrity of your building.

In many of Norm’s presentations, he has compared the similar shape of a greenhouse to that of an airplane wing. The basic challenge is that you want the opposite result. An airplane needs to efficiently get off the ground while a greenhouse needs to stay where it is.

The greater the distance over a surface is (wing or building), the greater that the lift is. When height is combined with distance, there is a multiplier effect. That is why a pilot will extend the flaps on a wing when taking off and landing.

It is the vacuum on the backside of a structure or a wing that causes the lift. The upward force of the vacuum on the backside is actually double what the leading edge force is.

This effect happens exponentially even when a small change is made. Many people have suggested, “I am only lifting the building two feet (or some other relatively small amount)”. They do not realize that doing that small lift is sometimes adding 50% or even doubling the wind load on the building.

These forces must especially be considered when a structure is oriented so that the prevailing winds hit the sides of the building. Structurally, it is preferable to have the structure facing into the prevailing winds. If the lay of the land or the logistics of how you work in the structure do not allow this, there are ways that the stress on the building can be minimized.

The least expensive way to stiffen a building is to add crossties (also known as collar ties). Crossties tie the left and right sides of the structure together so that the load is more balanced in how it affects the structure. Reducing the hoop spacing will automatically make the structure stronger with a greater ability to resist the effects of external forces.

Ultimately, however, a structure’s ability to deal with the aerodynamics of a situation is only as good as the anchoring provided. Anchors provide stability for resisting up, down and lateral forces. There is no such thing as too many anchors.

These things add to the structural ability to handle things but it does not help the cover. Extra steps must be implemented to secure the sides of the cover. This includes but is not limited to adding extra and longer screws for the cover fastening system.

A discussion on aerodynamics is not complete without discussing windbreaks. The only effective windbreak is trees. Trees restrict the speed of the wind coming through and therefore minimize aerodynamic stresses on a building. There is an optimal distance from a windbreak. This is determined by the size, density and texture of the trees involved. Since this is something that changes with age and season, there is not a perfect formula.

One last note on windbreaks. Another building, especially a bigger building is not an effective barrier. If anything, this building will significantly increase the aerodynamic stresses on your building. Wind will be swirling in one direction and then a slight shift in wind direction will reverse the direction of the swirling.

Please note: We are not engineers and as such can not give structural opinions. The above points are simply items that should be considered and come from 40+ years of experience of watching what wind does to buildings.